The present invention relates to the production of magnesium by the thermal reduction of magnesium oxide in the presence of a molten oxide slag. More particularly, this invention relates to the production of magnesium by contacting or reacting a metallic reducing agent with a molten calcium-silicon-aluminum-magnesium oxide slag.
Several processes for the production of magnesium by thermal reduction are known. These processes generally operate to react magnesium oxide with a metallic reducing agent such as silicon, aluminum, calcium or mixtures or alloys thereof. The reaction may take place in the solid state or in the liquid state.
The Pidgeon Process, described in U.S. Pat. No. 2,330,143, is a well-known solid state reaction process for the production of magnesium. In carrying out this process, a magnesium oxide ore, such as calcined dolomite, and ferrosilicon are formed into briquettes and charged to a gas-fired or electrically heated retort having a reaction zone and a water-cooled condensation zone. The retort is evacuated and heated so that the temperature in the reaction zone is about 1150.degree. C. Typically, the pressure in the reaction zone is less than 1 torr. Under these conditions, the ferrosilicon reacts with the magnesium oxide ore to produce magnesium vapor. The vapor so produced is conducted to the condensation zone, where it is condensed as a solid.
Another thermal reduction process utilizing a solid state reaction is described in U.S. Pat. No. 2,448,000 of Kemmer. This process is similar to the Pidgeon Process, but it utilizes aluminum as the reducing agent, and it also requires the addition of a moderating agent to the reaction zone. This moderating agent consists of aluminum nitride, a mixture of aluminum nitride, aluminum carbide and aluminum oxide or a mixture of ferrosilicon, aluminum nitride, aluminum carbide and aluminum oxide. In one embodiment of this process, there is used as a combined reducing agent and moderating agent "the dross which is obtained in melting and subsequently casting aluminum or aluminum alloys", provided that the dross contains about 0.5 to 10% by weight aluminum nitride.
A thermal reduction process for the production of magnesium by a liquid state reaction is described in U.S. Pat. No. 2,971,833. This process, called the Magnetherm Process, includes a reaction between a metallic reducing agent and a liquid mixture of oxides in a reaction zone which is heated by the electrical resistance of the mixture of oxides. In carrying out this process, a magnesium oxide ore, such as calcined dolomite, and a reducing agent comprised of silicon, ferrosilicon or an alloy of aluminum and ferrosilicon are charged to the reaction zone of a reaction-condensation system. Aluminum oxide is also added to the reaction zone and the composition of the total charge is controlled so that a particular liquid slag, a mixture of oxides of calcium, silicon, aluminum and magnesium, is formed and maintained in the reaction zone. The composition of the slag is controlled so that the molecular ratio of CaO to SiO.sub.2 is at least 1.8 (i.e., weight ratio is 1.68) and the molecular ratio of Al.sub.2 O.sub.3 to SiO.sub.2 is at least 0.26 (i.e., weight ratio is 0.44). The reaction is carried out at a temperature within the range of 1300.degree. to 1700.degree. C. and at a pressure of at least 1.5 torr. Preferably, the Magnetherm Process is operated at a pressure within the range of 5 to 20 torr. Under these conditions, the metallic reducing agent reacts with the calcium-silicon-aluminum-magnesium oxide slag to produce magnesium vapor. The vapor is conducted to the condensation zone where it is condensed as either a liquid or a solid.
Since the development of the Magnetherm Process, several thermal reduction processes for the production of magnesium by a liquid state reaction have been proposed. Like the Magnetherm Process, these processes include the use of a metallic reducing agent, and they require that the composition of the molten oxide slag in the reaction zone be controlled within prescribed limits. These processes operate under various temperature and pressure conditions. They utilize various reducing agents, and most of them require the addition of additives, such as aluminum oxide, to the reaction zone to achieve a liquid state reaction in the presence of a molten oxide slag of controlled composition.
Several of the more recent thermal reduction processes require that the liquid state reaction be carried out under a considerably higher absolute pressure than that of the Magnetherm Process. Thus, for example, U.S. Pat. No. 4,033,759 of Johnston et al describes a process in which the reaction is carried out under a system pressure within the range of 0.5 to 2 atmospheres (380 to 1520 torr). Several of the processes described in the U.S. patents of Avery require the maintenance of an inert gas in the reaction zone of the reaction-condensation system to provide the desired pressure conditions. For example, the process of U.S. Pat. No. 3,658,509 of Avery requires the maintenance in the reaction zone of an inert gas at a partial pressure within the range of 0.1 to 5 atmospheres (76 to 3800 torr). Avery's U.S. Pat. No. 3,698,888 describes a process which is carried out in the presence of an inert gas at a partial pressure within the range of 0.25 to 2 atmospheres (190 to 1520 torr).
A variety of slag compositions have been used in recent thermal reduction processes for the production of magnesium by a liquid state reaction. Most of the processes of Avery reportedly may be carried out in the presence of molten slags having broad compositional ranges. Thus, for example, Avery's U.S. Pat. No. 3,761,247 describes a process which may be carried out in the presence of a molten slag containing 0 to 70% by weight calcium oxide, 0 to 25% by weight aluminum oxide, 5 to 30% by weight magnesium oxide and 25 to 50% by weight silicon dioxide. Avery's U.S. Pat. Nos. 3,658,509, 3,681,053, 3,698,888 and 3,994,717 also describe processes which may be carried out in the presence of molten slags having broad compositional ranges. The slag described in U.S. Pat. No. 3,658,509 contains 10 to 60% by weight calcium oxide, 10 to 35% by weight aluminum oxide, 5 to 25% by weight magnesium oxide and 20 to 50 % by weight silicon dioxide. The slag described in U.S. Pat. No. 3,681,053 contains 10 to 60% by weight calcium oxide, 0 to 35% by weight aluminum oxide, 3 to 25% by weight magnesium oxide and 20 to 50% by weight silicon dioxide. The slag of U.S. Pat. No. 3,994,717 has the same compositional ranges as that of U.S. Pat. No. 3,681,053, except that the slag may contain 2 to 25% by weight magnesium oxide. The slag of U.S. Pat. No. 3,698,888 contains 0 to 65% by weight calcium oxide, 0 to 25% by weight aluminum oxide, 5 to 30% by weight magnesium oxide and 30 to 50% by weight silicon dioxide.
Several of the recent processes may be carried out in the presence of molten slags having relatively high concentrations of silicon dioxide. All of the processes of Avery mentioned in the preceding paragraph may be carried out in the presence of slags which contain up to 50% by weight silicon dioxide. In addition, Avery's U.S. Pat. No. 3,579,326 describes a process which may be carried out in the presence of a slag which contains a relatively high percentage of silicon dioxide and a relatively low percentage of calcium oxide. This slag contains 0 to 30% by weight calcium oxide, 15 to 35% by weight aluminum oxide, 5 to 25% by weight magnesium oxide and 25 to 50% by weight silicon dioxide.
Several of the recent processes are carried out in the presence of molten slags having relatively low concentrations of silicon dioxide. The slags which have relatively low concentrations of silicon dioxide usually have relatively high concentrations of aluminum oxide. For example, U.S. Pat. No. 3,782,922 of Avery describes a process which may be carried out in the presence of a slag containing 35 to 55% by weight calcium oxide, 35 to 65% by weight aluminum oxide, less than 5% by weight magnesium oxide and 0 to 10% by weight silicon dioxide. The U.S. patents of Johnston et al also describe processes which are carried out in the presence of molten slags having relatively low concentrations of silicon dioxide. Thus, U.S. Pat. No. 4,033,758 describes a slag containing 42 to 65% by weight calcium oxide, 11 to 38% by weight aluminum oxide, 1 to 11% by weight magnesium oxide and 5 to 19% by weight silicon dioxide. U.S. Pat. No. 4,033,759 describes a slag containing 30 to 65% by weight calcium oxide, 28 to 64% by weight aluminum oxide, 6 to 13% by weight magnesium oxide and less than 5% by weight silicon dioxide. The slag of U.S. Pat. No. 4,066,445 has the same compositional ranges as that of U.S. Pat. No. 4,033,759, except that the slag may contain 6 to 16% by weight magnesium oxide.
A variety of metallic reducing agents have been utilized in thermal reduction processes for the production of magnesium by a liquid state reaction. Many of these processes utilize reducing agents containing a significant amount of silicon. Some utilize silicon-rich alloys of aluminum and silicon or aluminum and ferrosilicon. Thus, for example, U.S. Pat. No. 3,681,053 of Avery describes a process which uses as a reducing agent an alloy containing about 80 to 99.75% by weight silicon, 0 to 20% by weight aluminum and 0.25 to 10% by weight iron. U.S. Pat. No. 3,579,326 of Avery describes a use as a reducing agent of an alloy containing 40 to 65% by weight silicon, 25 to 50% by weight aluminum and 0 to 20% by weight iron. Essentially the same reducing agent is used in the processes of Avery's U.S. Pat. No. 3,658,509. Avery's U.S. Pat. No. 3,994,717 discloses the use of a reducing agent having a composition similar to that described in Avery's U.S. Pat. No. 3,579,326. The '717 patent additionally mentions that scrap aluminum may be used to provide the aluminum component of the reducing agent. Avery's U.S. Pat. Nos. 3,698,888 and 3,761,247 describe uses of a reducing alloy containing 50 to 100% by weight silicon, 0 to 40% by weight aluminum and 0 to 15% by weight iron.
Some of the known processes employ reducing agents that are rich in aluminum. Thus, U.S. Pat. No. 3,782,922 of Avery describes a process which uses as a reducing agent aluminum or an aluminum alloy which contains at least 85% by weight aluminum. U.S. Pat. No. 4,033,759 and U.S. Pat. No. 4,066,445, both of Johnston et al, describe processes which use as a reducing agent aluminum having a purity of at least 80% by weight, and U.S. Pat. No. 4,033,758, also to Johnston et al, discloses a process utilizing an aluminum-silicon alloy as a reducing agent which contains from 15 to 75 wt. % aluminum.
Aluminum is a reactive metal, and it reacts at room temperature with a variety of acids, bases and other reagents. It is also quite reactive at the high temperatures required for the production of magnesium. As a matter of fact, aluminum is a more reactive reducing agent than silicon or ferrosilicon in a liquid state thermal reduction process for the production of magnesium, because it produces a higher vapor pressure of magnesium at a lower temperature. However, there are disadvantages to the use of aluminum as a reducing agent in such a process. Aluminum is generally more expensive than either silicon or ferrosilicon, and because of its high reactivity at high temperatures, aluminum can react not only with magnesium oxide, but also with the silicon dioxide in the molten oxide slag. This can result in the simultaneous production of magnesium, silicon monoxide and silicon, with the silicon appearing as an impurity in the magnesium product.
Accordingly, a commercially viable, low silicon thermal reduction process capable of using low-cost aluminum as a reducing agent would be most beneficial, if available.